1. Field of The Invention
This invention relates generally to a circuit and method for demodulating an amplitude-modulated (AM) or frequency-modulated (FM) signal, and more particularly is directed to the demodulating of a signal in the audio band which is amplitude- or frequency- modulated with video signal data.
2. Description of the Prior Art
It is known to effect communication of an image by means of an amateur radio communication system, a personal computer communication system, a still image video telephone system and the like. At the transmission side of each such communication system, one field of a standard video signal picked up by a video camera is analog-to-digital (A/D) converted and the resulting data is written in a memory. The image data stored in the memory is read out at a relatively slow speed and digital-to-analog (D/A) converted into an AM or FM signal or the like in the audio band which is then transmitted, for example, through a telephone line or the like, to the receiver side of the system. At the receiver side, the original image data is demodulated from the received AM or FM signal, and then A/D converted and written in a memory. The image data stored in the memory at the receiver side is repeatedly read out and D/A converted to provide a video signal at a standard synchronizing frequency, and such video signal constituting a repeated field is supplied to a cathode ray tube (CRT) display for reproducing a still image thereon.
In the above described image communication system, the AM or FM signal which is transmitted and received may have any one of a variety of signal formats. For example, the transmitted and received signal may have the format of the signal S.sub.d shown in FIG. 1. More specifically, when an image transmission is to be effected from the transmission side, a level reference signal REFS is initially delivered as the signal S.sub.d for a period T.sub.1 of, for example, 1 second. Such level reference signal REFS is employed for correcting the level of the signal S.sub.d as received at the receiver side of the system. In order to serve the foregoing purpose, the level reference signal REFS is desirably a signal S.sub.1 having a frequency f.sub.1 of 2,000 Hz and a predetermined constant level or amplitude. The period T.sub.1 of the level reference signal REFS is followed by a blank or non-signal period T.sub.2 of, for example, 0.2 seconds. The transmitted signal S.sub.d further includes a period T.sub.3, for example, equivalent to the time required for 160 cycles of the signal S.sub.1, or approximately 0.08 seconds. During each period T.sub.3 following the blank period T.sub.2 a start STRT signal is transmitted as the signal S.sub.d. Such signal STRT is a synchronizing or marker signal indicating that image data is to be transmitted subsequent thereto. The signal STRT is, for example, a 160 cycle burst of the signal S.sub.1 having a fixed level or amplitude. During a period T.sub.4 following the period T.sub.3 of the signal STRT, the image data corresponding to one field is transmitted, for example, a carrier signal similar to the signal S.sub.1 is amplitude-modulated by the image data and the resulting AM signal S.sub.a is transmitted.
Such image data may be of 4 bits for representing the gradation or luminance of a respective pixel of one field of the video signal. As shown in FIG. 2, each cycle of the signal S.sub.a represents the image data (4 bits) for a respective pixel, and the amplitude of each cycle of the signal S.sub.a is AM-modulated in accordance with an analog value represented by the image data for the corresponding pixel. The amplitude modulation of the signal S.sub.1 for providing the transmitted AM signal S.sub.a is restricted so that the amplitude of the signal S.sub.a is a minimal value greater than 0 even when the image data is "0000" which corresponds to the white level of the video signal. Since the minimal amplitude of the AM signal S.sub.a is greater than 0, the signal S.sub.a will not be interrupted during the period T.sub.4 even when the AM signal S.sub.a is at its minimal amplitude and, therefore, the signal S.sub.1 exists as a carry signal. On the other hand, the maximal amplitude of the AM signal S.sub.a which is obtained when the image data is "1111" is made equal to the predetermined amplitude of the level reference signal REFS.
It will be appreciated that, during the period T.sub.4, the AM signal S.sub.a is transmitted for a number of cycles of the signal S.sub.1 equal to the number of pixels in the video field to be transmitted. For example, in the case of a video field comprised of 160 pixels.times.100 pixels, the signal S.sub.a is transmitted for 16000 cycles of the signal S.sub.1, whereupon the signal S.sub.d is terminated.
The described format for the signal S.sub.d allows the latter to be contained within the audio band, so that such audio band can be used for transmitting and receiving image data, for example, by means of an amateur radio communication system, a personal computer communication system, or a still image video telephone system. Furthermore, the described format of the signal S.sub.d, when used to represent a video signal recorded by an electronic still camera, permits such video signal to be transmitted and received through a telephone cable. It is also possible to use an audio tape recorder for storing video images converted to the described signal format.
In general, the demodulation of an AM signal has been heretofore effected by means of an envelope detecting circuit and a synchronizing detecting circuit which are provided with a low-pass filter at a rear stage thereof for detecting the peak level of each cycle of the AM signal and for taking out the detected peak level as a demodulated output. When using the foregoing arrangement for demodulating the image data from the above described AM signal S.sub.a it is intended that the demodulated output or image data will pass through the low pass filter.
However, if the time constant of the low pass filter is too large, the peak value of each cycle of the AM signal S.sub.a is held over to the next cycle so that it is impossible to accurately take out the peak value of each cycle. In that case, since each cycle of the AM signal S.sub.a represents the luminance or gradation of a respective pixel in the reproduced image, the luminance of each pixel of the reproduced image is influenced by the luminance of the adjacent pixels in the event that the time constant of the low pass filter is selected to be large.
On the other hand, if the time constant of the low pass filter is small for avoiding the above described problem, noise included in the AM signal S.sub.a will readily pass through the low pass filter and thereby cause deterioration of the quality of the reproduced image. Further, unless the AM signal S.sub.a is maintained sufficiently below a reference level in either the envelope detecting circuit or the synchronizing detecting circuit of the known demodulation circuit, the demodulated output or image data, when being A/D converted, may exceed the dynamic range of the A/D converter used therefore. Therefore, it is necessary to effect substantially accurate automatic gain control (AGC) or level correction of the AM signal S.sub.a by means of the preceding level reference signal REFS. However, such substantially accurate AGC requires a relatively complicated and costly construction. Alternatively, the A/D converter for the demodulated output may be provided with a relatively wide dynamic range, or a longer bit length, which again increases the cost of the system.